Gas insensitive mass flow control systems and methods

ABSTRACT

Gas insensitive systems and methods for controlling the mass flow rate of a gas through a primary conduit are disclosed. The method includes producing, in a secondary conduit, an assessment flow that has a changing pressure; calculating a first measure of a flow rate of the assessment flow based upon a rate-of-change of the pressure of the assessment flow; and measuring the assessment flow with a mass flow meter that is affected by the composition of the gas to produce a second measure of the assessment flow. An adjustment signal is generated based upon a difference between the first measure and the second measure, and a gas-corrected flow signal is generated by adjusting a measured-flow signal of a mass flow controller with the adjustment signal. The gas-corrected flow signal is used by the mass flow controller to control a flow rate of the gas through the primary conduit.

CLAIM OF PRIORITY UNDER 35 U.S.C. §119

The present application for patent claims priority to ProvisionalApplication No. 62/272,315 entitled “GAS INSENSITIVE MASS FLOWCONTROLLERS AND METHODS” filed 29 Dec. 2015, and assigned to theassignee hereof and hereby expressly incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to systems and methods for mass flowcontrol. In particular, but not by way of limitation, the presentinvention relates to systems and methods for gas insensitive mass flowcontrol.

BACKGROUND OF THE INVENTION

A typical mass flow controller (MFC) is a device that sets, measures,and controls the flow of a fluid (e.g., a gas or a liquid). An importantpart of an MFC is a sensor that measures the mass flow rate of a fluidflowing through the device. The MFC compares an output signal of thesensor with a predetermined set point and adjusts a control valve tomaintain the mass flow rate of the gas at the predetermined set point.

The mass flow sensor of an MFC is typically calibrated against aprecision mass flow meter so that the output signal of the MFC sensor isadjusted (using calibration data) to match the measured flow of theprecision mass flow meter. The calibration of mass flow controllers istypically performed by MFC manufacturers with a calibration gas,typically nitrogen (N₂). Often, the obtained calibration data isgas-dependent—especially in the context of thermal mass flow sensors. Asa consequence, when the MFC is operating with a gas other than thecalibration gas, the calibration data may result in the MFC providing aflow rate that does not match the desired set point.

Due to the tendency of mass flow controllers to be inaccurate when thegas that is being controlled varies from the calibration gas, on-toolflow verification systems, such as the system depicted in FIG. 7, havebeen utilized to provide a reference flow measurement that is comparedagainst the measurement of the MFC. These flow verification systems mayutilize periodic pressure based measurements of flow that are comparedto the measurements of the MFC. For example, periodic rate-of-rise orrate-of-decay measurements are known to be utilized to determine whetherthe measured flow of an MFC has departed from the actual flow by virtueof variations in the composition of the fluid being controlled.

But these rate-of-rise and rate-of-decay systems merely providereference information (e.g., as an alarm), and they may interfere withthe flow of the fluid being controlled. As shown in FIG. 7 for example,a depicted rate of decay system is disposed in the same flow path as amass flow controller; thus, interfering with the flow of the controlledfluid. Moreover, if the flow rate of the fluid being controlled is veryhigh (e.g., exceeding 100 liters per minute), a very large containmentchamber would be required for the rate-of-decay system of FIG. 7 tomeasure flow accurately. Thus, rate-of-rise and rate-of-decay systemsare not typically used in connection with high flow rate systems.

SUMMARY OF THE INVENTION

Exemplary embodiments of the present invention that are shown in thedrawings are summarized below. These and other embodiments are morefully described in the Detailed Description section. It is to beunderstood, however, that there is no intention to limit the inventionto the forms described in this Summary of the Invention or in theDetailed Description. One skilled in the art can recognize that thereare numerous modifications, equivalents and alternative constructionsthat fall within the spirit and scope of the invention as expressed inthe claims.

According to an aspect, a mass flow control system includes a primaryconduit for directing a flow of a gas; an adjustment system configuredto divert a portion of the gas from the primary conduit to a secondaryconduit and provide an adjustment signal that changes when a compositionof the gas changes. The adjustment system includes a pressurizationsystem to produce an assessment flow in the secondary conduit that has achanging pressure; a rate-of-change flow meter to provide, based upon arate of change of a pressure of the assessment flow, a first measure ofan assessment flow rate of the assessment flow. The adjustment systemalso includes a mass flow meter disposed to provide a second measure ofthe assessment flow rate of the assessment flow through the secondaryconduit and an adjustment component to generate the adjustment signalbased upon a difference between the first measure and the second measureof the assessment flow rate. A mass flow controller is operativelycoupled to the primary conduit to control a primary flow rate of thegas. The mass flow controller includes a mass flow measurement systemconfigured to generate a measured-flow signal based upon a flow rate ofthe gas through the primary conduit; a correction module configured toadjust the measured-flow signal with the adjustment signal to generate agas-corrected flow signal; and a controller configured to control thevalve so the gas-corrected flow signal indicates a flow rate of the gasthrough the primary conduit is equal to a set point.

According to another aspect, a method for controlling a flow rate of agas includes controlling the flow rate of the gas through a primaryconduit with a mass flow controller, diverting a portion of the gas to asecond conduit, producing an assessment flow in the second conduit thathas a changing pressure, calculating a first measure of a flow rate ofthe assessment flow based upon a rate-of-change of the changing pressureof the assessment flow, measuring the flow rate of the assessment flowwith a mass flow meter that is affected by the composition of the gas toproduce a second measure of the assessment flow, and generating anadjustment signal based upon a difference between the first measure andthe second measure of the assessment flow rate. A measured-flow signalof the mass flow controller is adjusted with the adjustment signal togenerate a gas-corrected flow signal, and a valve of the mass flowcontroller is controlled so the gas-corrected flow signal indicates aflow rate of the gas through the primary conduit is equal to a setpoint.

BRIEF DESCRIPTION OF THE DRAWINGS

Various objects and advantages and a more complete understanding of thepresent invention are apparent and more readily appreciated by referenceto the following Detailed Description when taken in conjunction with theaccompanying Drawings wherein:

FIG. 1 is a block diagram depicting an embodiment of a mass flow controlsystem;

FIG. 2 is a flowchart depicting a method that may be traversed inconnection with the mass flow control system of FIG. 1;

FIG. 3 is a block diagram illustrating further details of an embodimentof the mass flow controller depicted in FIG. 1;

FIG. 4A is a block diagram depicting an embodiment of the mass flowmeter of FIG. 1;

FIG. 4B is a block diagram depicting another embodiment of the mass flowmeter of FIG. 1;

FIG. 5 is a block diagram depicting an embodiment of the rate-of-changemeter depicted in FIG. 1;

FIG. 6 is a diagram depicting physical components that may be utilizedto realize one or more components described herein; and

FIG. 7 is a block diagram depicting aspects of prior art.

DETAILED DESCRIPTION

The word “exemplary” is used herein to mean “serving as an example,instance, or illustration.” Any embodiment described herein as“exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments. It is also noted that, as usedherein and in the appended claims, the singular forms “a,” “an,” and“the” include plural referents unless the context clearly dictatesotherwise.

Referring to FIG. 1, shown is a gas insensitive mass flow control system100 including a mass flow controller 102 and an adjustment system 104.In general, the mass flow controller 102 operates to control a primaryflow rate of a gas that is provided via a primary conduit 106 to anothersystem (not shown)(e.g., a fuel cell system) that uses the gas. Asdescribed further herein, an accuracy of a mass flow measurement system(not shown in FIG. 1) of the mass flow controller 102 varies based on acomposition of the controlled gas. More specifically, if the chemicalcomposition of the gas fluctuates, such as through contamination of thegas, the accuracy of the control system of the mass flow controller 102(without an adjustment signal 108 from the adjustment system 104) willvary.

As one of ordinary skill in the art will readily appreciate, the massflow controller 102 may be calibrated for accuracy with a calibrationgas, but if the composition of the controlled gas is different than thecalibration gas, the sensing components within the mass flow controller102 will provide flow signals that inaccurately represent the flow rateof the gas. In some implementations for example, the mass flowcontroller 102 utilizes thermal mass flow sensor technology, which isaffected by the composition of the controlled gas.

The adjustment system 104 generally operates to provide the adjustmentsignal 108 to the mass flow controller 102 when a composition of the gaschanges so that the mass flow controller 102 may accurately meet a setpoint for the flow of the gas. As shown, the adjustment system 104 isoperatively coupled to the primary conduit 106 via a secondary conduit110. The adjustment system 104 includes a pressurization system thatincludes an upstream valve 112, a downstream valve 114, and anassessment-flow controller 115 that generally operates to produce anassessment flow 116 through a portion 119 of the secondary conduit 110that includes a known volume 120.

The adjustment system 104 also includes a rate-of-change flow meter 118that provides a first measure 121 of the flow rate of theassessment-flow 116, which is substantially independent of thecomposition of the gas, based upon a rate-of-change of a pressure of theassessment flow 116. In the embodiment depicted in FIG. 1, therate-of-change flow meter 118 includes a pressure sensor 125 and atemperature sensor 132, and is disposed and configured as arate-of-decay flow meter in which the upstream valve 112 and adownstream valve 114 are controlled to pressurize the gas in the knownvolume 120 and then release the pressurized gas to create the assessmentflow 116, which has a pressure that is decreasing as the gas exits theknown volume 120. When the gas is released, the assessment flow 116 ismeasured by both the rate-of-change flow meter 118 and a mass flow meter122.

Similar to the mass flow controller 102, the accuracy of the mass flowmeter 122 in this embodiment is also affected by the composition of thegas. As a consequence, the mass flow meter 122 provides a second measure124 of the flow rate of the assessment-flow 116 that isgas-composition-dependent. In some implementations for example, the massflow meter 122 and the mass flow controller 102 utilize the same, orvery similar, flow-rate-sensing technology so that anygas-composition-related effects on the accuracy of the mass flowcontroller 102 are mirrored by the mass flow meter 122. In someembodiments, both the mass flow controller 102 and the mass flow meter122 utilize thermal mass flow sensing technologies.

As shown, an adjustment component 126 is disposed to receive the firstmeasure 121 (gas-composition-independent measure) of the assessment flowrate from the rate-of-change flow meter 118 and the second measure 124(gas-composition-dependent measure) of the assessment flow rate from themass flow meter 122, and in response, the adjustment component 126generates and provides the adjustment signal 108 to the mass flowcontroller 102. In turn, the mass flow controller 102 uses theadjustment signal 108 (as discussed further herein) to render thecontrol loop of the mass flow controller 102 more accurate.

More specifically, any difference between the first measure 121(gas-composition-independent measure) and the second measure 124(gas-composition-dependent measure) indicates the composition of the gashas changed, and that neither the mass flow meter 122 nor the mass flowcontroller 102 are providing accurate flow rate measurements; thus, theadjustment signal 108 is indicative of changes in the composition of thegas, and hence, an inaccuracy of the mass flow meter 122. And becausethe changes in the composition of the gas similarly affect both the massflow meter 122 and the mass flow measurement system of the mass flowcontroller 102, the adjustment signal 108 (generated based on theinaccuracy of the mass flow meter 122) may be used to correct forinaccuracies of the mass flow controller 102.

It should be recognized that the depiction of the components in FIG. 1is logical to depict functional aspects of the present embodiment, andit is not intended to convey a particular distribution of physicalcomponents. More specifically, the depicted components may be realizedby an integrated combination of discrete hardware constructs or may berealized by a collection of distributed hardware constructs. Forexample, the functions of the rate-of-change flow meter 118, theadjustment component 126, and the assessment-flow controller 115 may berealized (at least in part) by common hardware components. This commonhardware may include a processor, memory, and non-transitoryprocessor-executable instructions embodied in non-transitory memory. Asanother example, the rate-of-change flow meter 118, the adjustmentcomponent 126, and the assessment-flow controller 115 may be realized bydiscrete hardware components such as field programmable gate arrays,application specific integrated circuits, transistor logic, and/or oneor more processors in connection with memory and non-transitoryprocessor-executable instructions embodied in non-transitory memory.

It should also be recognized that the known volume 120 depicted in FIG.1 is intended to convey a known volume that the gas occupies in thesecondary conduit—it is not necessarily intended to convey a separatecontainment vessel. In other words, the known volume 120 may simply bethe volume of the second conduit 110 in which the diverted portion ofthe gas is pressurized. Moreover, one of ordinary skill in the art willrecognize that some or all of the components of the adjustment system104 may be implemented in the same housing as the mass flow controller102.

In many embodiments, the mass flow controller 102 may have an operatingrange of at least about 100 liters per minute, and in some embodimentsmay have an operating range of over about 500 liters per minute. Incontrast, the adjustment system 104 may have an operating range of nomore than about 800 standard cubic centimeters per minute. In yet otherembodiments, the adjustment system 104 has an operating capacity of nomore than about 300 standard cubic centimeters per minute. In stillother embodiments, the adjustment system 104 has an operating capacityof no more than about 100 standard cubic centimeters per minute.Operating with such a small volume of gas flow allows the adjustmentsystem 104 to exhibit fast response times to changes in the compositionof the gas without substantially interrupting the flow of the gasthrough the primary conduit 106. Put another way, the mass flowcontroller 102 may have an operating range of between about 120 and6,000 times over the operating range of the adjustment system 104.

For the purpose of this disclosure, the term “about” should beunderstood to mean within a range that is either customary for theindustry, or within standard manufacturing or process tolerances,whichever is greater. As one example, in some industries, a range of +or −20% is customary, although a manufacturing tolerance might be less.

While referring to FIG. 1, simultaneous reference is made to FIG. 2,which is a flow chart depicting a method that may be traversed inconnection with the system 100 depicted in FIG. 1. As shown, the flowrate of a gas through the primary conduit 106 is controlled by the massflow controller 102 (Block 200). As discussed above, the gas may bemethane gas that is provided to a fuel cell system, but otherembodiments are certainly contemplated. In many implementations, theassessment-flow controller 115 generally maintains both the upstreamvalve 112 and the downstream valve 114 in a closed position, and todetermine whether the composition of the gas has changed, and hence,whether the accuracy of the mass flow controller 102 may have changed, aportion of the gas may be diverted to the adjustment system 104 byopening the upstream valve 112 (Block 202). The verification system thenproduces the assessment flow 116 that has a changing pressure (Block204).

Although the rate-of-change flow meter 118 may be implemented usingeither a rate-of-rise or rate-of-decay meter, the mass flow meter 122depicted in FIG. 1 is disposed downstream relative to the known volume120 so that the rate-of-change flow meter 118 is disposed to operate asa rate-of-decay meter system. And as implemented in FIG. 1, thepressurization system (including the upstream valve 112, a downstreamvalve 114, and the assessment-flow controller 115) operates to producethe assessment flow 116 so that the assessment flow 116 has a decreasingpressure.

More specifically, to operate as a rate-of-decay system, the downstreamvalve 114 is closed while the upstream valve 112 is open to enable thediverted portion of the gas to occupy the known volume 120 of thesecondary conduit 110 under pressure. The upstream valve 112 is thenclosed to isolate the diverted and pressurized portion of the divertedgas from the primary conduit 106 in the known volume 120. Theassessment-flow controller 115 then opens the downstream valve 114 whilethe upstream valve 112 is closed to produce the assessment flow 116 thathas a decreasing pressure as the diverted portion of the gas is releasedfrom the known volume 120.

As shown in FIG. 2 both the first measure 121 of a flow rate of theassessment flow (based upon a rate of change of a pressure of the gas)and the second measure 124 of the assessment flow 116 are obtained(Blocks 206 and 208), and the adjustment 108 signal is generated basedupon a difference between the first measure 121 and the second measure124 of the assessment flow 116 (Block 210). In the embodiment depictedin FIG. 1, the rate-of-change flow meter 118 calculates the firstmeasure 121 of the flow rate of the assessment flow 116 based upon arate of change of a pressure of the gas, and the mass flow meter 122measures the flow rate of the assessment flow with a sensor that isaffected by the composition of the gas to produce the second measure 124of the assessment flow 116 (Block 208).

The adjustment component 126 then generates the adjustment signal 108based upon a difference between the first measure 121 and the secondmeasure 124 of the assessment flow 116 (Block 210). In one embodimentfor example, the adjustment component 126 calculates a differencebetween the first measure 121 and the second measure 124 and divides thedifference by the second measure 124 (the gas-composition-dependentmeasure) to obtain the adjustment signal 108, which is then provided tothe mass flow controller 102 to correct a measured flow signal of thecontrol loop of the mass flow controller 102. In the embodiment depictedin FIG. 1, the effects of the change of the composition of the gas onthe mass flow meter 122 generally mirror the effects on the mass flowcontroller 102. As a consequence, the adjustment signal 108 that isgenerated based upon an inaccuracy of the mass flow meter 122 may beused to correct errors of the mass flow controller 102.

Referring now to FIG. 3, shown is an exemplary mass flow controller(MFC) 302 that may be utilized as the mass flow controller 102 describedwith reference to FIG. 1. It should be recognized, however, that the MFC302 is only an example of the types of mass flow controllers that may beimplemented as the mass flow controller 102, and that other types ofmass flow controllers may be utilized as the mass flow controller 102 inthe system 100 depicted in FIG. 1. It should also be recognized that theillustrated arrangement of these components is logical and not meant tobe an actual hardware diagram. Thus, the components can be combined,further separated, deleted and/or supplemented in an actualimplementation. As one of ordinary skill in the art will appreciate, thecomponents depicted in FIG. 3 may be implemented in hardware, firmware,software, or any combination thereof. Moreover, in light of thisspecification, the construction of each individual component is wellknown within the skill of those of ordinary skill in the art.

As depicted, a base of the MFC 302 includes bypass 310 through which agas flows. The bypass 310 directs a constant proportion of gas throughmain path 315 and sensor tube 320. As a consequence, the flow rate ofthe fluid (e.g., gas or liquid) through the sensor tube 320 isindicative of the flow rate of the fluid flowing through the main path315 of the MFC 302.

In this embodiment, the sensor tube 320 is a small-bore tube that ispart of a thermal mass flow sensor 325 of the MFC 302. In general, thethermal mass flow sensor 325 is configured to provide an output signal330, which is a digital signal that is indicative of the flow ratethrough the sensor tube 320, and hence, indicative of the flow ratethrough the main path 315 of the MFC 302. As one of ordinary skill inthe art will readily appreciate, a variety of different technologiesincluding bridge-disposed resistance-thermometer elements (e.g., coilsof conductive wire), resistance temperature detectors (RTD), andthermocouples in connection with analog, analog-to-digital, and digitalprocessing technologies may be used realize the thermal mass flow sensor325. As shown, the output signal 330 in this embodiment is received by acalibration component 332 that tunes the signal 330 by adjusting thesignal 330 using predetermined calibration data 334 that was generatedduring a calibration process (e.g., by a manufacturer of the MFC 302)using a calibration gas (e.g., nitrogen) so that a measured-flow signal336 provides an accurate representation of a flow rate of thecalibration gas through the MFC 302 under a variety of operatingconditions (e.g., under a variety of temperatures and set points).

But when the composition of the gas changes from the calibration gas,the measured-flow signal 336 may become inaccurate, i.e., an accuracy ofthe measured-flow signal 336 is dependent upon the composition of thecontrolled gas. As a consequence, and as shown in FIG. 2, in thisembodiment the adjustment signal 108 from the adjustment system 104 isutilized by a correction module 338 to adjust the measured-flow signal336 to generate a gas-corrected flow signal 339 (Block 212).

The controller 340 along with the thermal mass flow sensor 325, thecalibration component 332, and the correction module 338 in thisembodiment are part of a control loop that operates to generate acontrol signal 345 to control a position of the control valve 350 basedupon the gas-corrected flow signal 339 to provide a flow rate that isindicated by a set point signal 355. In other words, the controller 340is configured to control the valve 350 so the gas-corrected flow signal339 indicates the primary flow rate of the gas through the primaryconduit 106 is equal to a set point (as indicated by the set pointsignal 355). For example, the flow rate may exceed 100 liters perminute, but the flow rate will vary depending upon the set point signal355. The control valve 350 may be realized by a piezoelectric valve orsolenoid valve, and the control signal 345 may be a voltage (in the caseof a piezoelectric valve) or current (in the case of a solenoid valve).Although not depicted, the controller 340 also utilizes temperature andpressure inputs to more accurately control the flow rate. Both pressureand temperature sensors and corresponding implementations in the contextof mass flow controllers are well known to those of ordinary skill inthe art, and as a consequence, details of temperature and pressuresensor systems are not included herein.

Referring next to FIGS. 4A and 4B, shown are block diagrams depictingexemplary mass flow meters 422A and 422B, respectively, which may beutilized to realize the mass flow meter 122 described with reference toFIG. 1. Although not required, in the context of the embodimentdescribed with reference to FIG. 1, several aspects of the mass flowmeters 422A, 422B utilize the same technology as the mass flowcontroller 102 depicted in FIG. 1. More specifically, both the sensors325, 425 may be thermal mass flow sensors, and the bypass 410 directs aconstant proportion of the assessment flow 116 through a main path 415and sensor tube 420 in the same way the bypass 310 directs a constantproportion of the primary gas flow through the main path 315 and sensortube 320 of the mass flow controller 302. As a consequence, the massflow meters 422A, 422B closely represent the mass flow measurementsystem 328 of the mass flow controller 302; thus, changes in the sensorsignal 330 of the mass flow controller 302 due to changes in the gascomposition will similarly occur to the sensor signal 430 of the massflow meters 422A, 422B.

As depicted, the mass flow meters 422A, 422B may also have a calibrationcomponent 432 and calibration data 434 that function in the same way asthe calibration component 332 and calibration data 334 of the mass flowcontroller 302 to provide a calibrated flow signal that is agas-composition-dependent measure 124 of the assessment flow 116.

The mass flow meter 422B depicted in FIG. 4B differs from the mass flowmeter 422A in FIG. 4A in that it includes a control valve 350 to operateas a restriction to ensure that a flow rate of the assessment flow 116is within a range of the sensor 425 and to create a pressure at thesensor 425 that is close to a pressure of the gas in the known volume120 (as opposed to the pressure at an outlet of the adjustment system104, which may be a vacuum). A controller 440 of the mass flow meter422B may also receive a signal 455 from the assessment-flow controller115 to open and close the control valve 350; thus the control valve 350may replace the downstream valve 114, i.e., the control valve 350 may becontrolled in the same way as the downstream valve 114 (described withreference to FIG. 1) to produce the assessment flow 116. In otherembodiments, flow restriction may be created by a control valve (or anyother flow restriction component that limits flow through the mass flowmeter 422) located anywhere between the known volume 120 and an outletof the adjustment system 104.

Referring next to FIG. 5, shown is a block diagram of a rate-of-changemeter system 518 that may be utilized to implement the rate-of-changemeter system 118 depicted in FIG. 1. As shown, the rate-of-change metersystem 518 includes a flow calculation component 520 that provides thefirst measure 121 (a gas-composition-independent measure) of theassessment flow 116. And as shown, a rate-of-pressure-change component522 receives a pressure signal 524 from the pressure sensor 125, and inresponse, provides a rate-of-change signal 528 to the flow calculationcomponent 520. In addition, the flow calculation component 520 receivesa temperature signal 530 from a temperature sensor 132.

The depiction of the components in FIG. 5 is logical to depictfunctional aspects of the rate-of-change component 518 and is notintended to be a hardware representation. More specifically, thedepicted flow calculation component 520 and rate-of-pressure-changecomponent 522 may be realized by hardware or hardware in connection withsoftware. As one of ordinary skill in the art will appreciate forexample, the rate-of-pressure-change component 522 may be realized by acombination of analog and digital components to sample the pressuresignal 524 (e.g., a voltage signal), and store digitized samples of thepressure signal 524 to enable the rate-of-change of the pressure in theknown volume 120 to be calculated based upon changes to the pressure ofthe assessment flow 116 over time.

In operation, the flow calculation component 520 calculates the firstmeasure 121 (the gas-composition-independent measure) of the assessmentflow 116 based upon the rate-of-change signal 528, the temperaturesignal 530, and the known volume 120. For example, the flow calculationcomponent 520 may use the ideal gas law to calculate the flow rate ofthe assessment flow 116. More specifically, the assessment flow 116 maybe calculated as:

${{AF} = {\frac{d\left( \frac{p}{T} \right)}{dt} \star \frac{T_{s} \star V}{P_{s} \star R}}},$

wherein

-   AF is the assessment flow;-   T and P are gas temperature and gas pressure measurements,    respectively;-   T_(s) and P_(s) are standard temperature (273.15 K) and standard    pressure (101.3 kPa), respectively;-   V is the known volume; and-   R is the universal gas constant (8.31441 J K⁻¹).

Referring next to FIG. 6, shown is a block diagram of a computing system600 depicting physical components that may be utilized to realize thegas insensitive mass flow control system 100 described with reference toFIG. 1. As shown, a display portion 612, and a nonvolatile memory 620are coupled to a bus 622 that is also coupled to a random access memory(“RAM”) 624, a processing portion (which includes N processingcomponents) 626, a collection of analog outputs 628, and a collection ofanalog inputs 630. Although the components depicted in FIG. 6 representphysical components, it should be recognized that the depicted computingsystem may be replicated and distributed to implement the componentsdepicted in FIG. 1.

The display portion 612 generally operates to provide a presentation ofcontent to a user, and in several implementations, the display isrealized by an LCD or OLED display. In general, the nonvolatile memory620 functions to store (e.g., persistently store) data and executablecode including non-transitory processor-executable code that isassociated with the functional components depicted in FIG. 1. In someembodiments for example, the nonvolatile memory 620 includes bootloadercode, software, operating system code, file system code, and code tofacilitate the method described with reference to FIG. 2.

In many implementations, the nonvolatile memory 620 is realized by aflash memory (e.g., NAND or ONENAND™ memory), but it is certainlycontemplated that other memory types may be utilized as well. Althoughit may be possible to execute the code from the nonvolatile memory 620,the executable code in the nonvolatile memory 620 is typically loadedinto the RAM 624 and executed by one or more of the N processingcomponents in the processing portion 626.

The N processing components in connection with the RAM 624 generallyoperate to execute the instructions stored in the nonvolatile memory 620to effectuate the functional components depicted in FIG. 1. For example,the assessment-flow controller 115, and logical aspects of therate-of-change flow meter system 118, mass flow controller 102, massflow meter 122, and adjustment component 126 may be realized by one ormore of the N processing components in connection with non-transitoryprocessor-readable code that is executed from the RAM 624. And thecalibration data 334, 434 may be stored in the non-volatile memory 620.

An interface component 632 generally represents one or more componentsthat enable a user to interact with the gas insensitive mass flowcontrol system 100. The interface component 632, for example, mayinclude a keypad, touch screen, and one or more analog or digitalcontrols, and the interface component 632 may be used to translate aninput from a user into the set point signal 355. And a communicationcomponent 634 generally enables the gas insensitive mass flow controlsystem 100 to communicate with external networks and devices includingthe external processing components (e.g., a fuel cell system). One ofordinary skill in the art will appreciate that the communicationcomponent 634 may include components (e.g., that are integrated ordistributed) to enable a variety of wireless (e.g., WiFi) and wired(e.g., Ethernet) communications.

Exemplary Fuel Cell Application

A fuel cell is a device that converts the chemical energy from a fuelinto electricity through a chemical reaction with oxygen or anotheroxidizing agent. Fuel cells are different from batteries in that theyrequire a continuous source of fuel to sustain the chemical reaction,whereas in a battery the chemicals present in the battery react witheach other to generate an electromotive force. Fuel cells can produceelectricity continuously for as long as the fuel is supplied.

A typical mass flow controller may be used to approximately maintain theflow of a fuel to a set point. But in practice, the chemical compositionof the supply gas fluctuates, such as through contamination of the gas,and affects the efficiency of the fuel cell. The typical mass flowcontroller, not being responsive to these fluctuations, allows a lessthan optimized energy output to result. To counteract this change inenergy output, the fuel cell industry is generally limited to measuringthe fuel cell energy output and, based on the measurements, adjustingthe flow of gas delivery. This process does not maximize the energyefficiency of the fuel cells because it causes delays in responding tochanges in the composition of the supply gas.

In the context of prior art fuel cell systems, if the primary gas becamecontaminated, affecting energy output of the fuel cell, the systems weredisabled, and recalibrated. This practice leads to undesirableinefficiencies in the system, and to overcome these deficiencies, thegas insensitive mass flow control system 100 may be used as a systemthat is insensitive to the gas species. Beneficially, the system 100provides a fuel cell operator with the ability to adjust a set point(with the set point signal 355) of the mass flow controller 102 inresponse to changes in the process gas without interrupting plantoperations.

The mass flow controller 102 is responsive to the adjustment signal 108,and may control flow of the primary gas G_(p) (e.g., methane) to anaccuracy of within 0.5% of the set point. The system 100 thus overcomesthe problem of impurities, including butane and other hydrocarbons,being introduced to the primary gas G_(p). This responsiveness maximizesefficiency of a fuel cell system and eliminates the need for shuttingdown the fuel cell process to calibrate the mass flow controller 102 inresponse to a new gas species.

For example, at start-up, the calibration data 334 of the mass flowcontroller 102 may be generated to calibrate the mass flow controller102 relative to methane gas. After start-up, however, the supply ofmethane may become tainted with impurities; thus, effectively creating anew gas species as the primary gas G_(p), and causing the readings ofthe mass flow controller 102 to be inaccurate—not because of a defect inthe mass flow controller 102, but because of a change in the propertiesof the fluid passing through the mass flow controller 102.

Using the adjustment system 104 as described herein, a deviation in thecomposition of the controlled fuel from an expected or desiredcomposition will prompt a change in the adjustment signal 108. The massflow controller 102, in turn, is responsive (as discussed above) to theadjustment signal 108 to account for the impurities.

It should be understood that the above adjustment process may berepeated continuously or periodically as desired. For example, if theprimary fluid or gas G_(p) is being drawn from a storage vessel, theoperator may assume that the primary fluid or gas G_(p) does not changeover time, and a single adjustment step when the sealed storage vesselis initially brought on-line may be all that is necessary. In contrast,if the primary fluid or gas G_(p) is being drawn from a public utilitiesline, the operator may assume that the primary fluid or gas G_(p)changes often over time, and regular verification and adjustment stepsmay be necessary, such as every several minutes, hours, days, etc. Asanother example, such as where the fuel cell energy generation processis particularly sensitive to fluctuations in the primary fluid or gasG_(p), the operator may wish to perform continuous adjustments tomaximize efficiency of the fuel cell energy generation.

It should be reiterated that the adjustment system 104 may have a muchsmaller capacity or chamber than the mass flow controller 102.Specifically, while the mass flow controller 102 and primary conduit 106have an operating capacity of at least about 100 liters per minute, or,in some embodiments, an operating capacity of at least about 500 litersper minute, the adjustment system 104 has an operating capacity of nomore than about 800 standard cubic centimeters per minute.

In some embodiments, the assessment flow 116 is released to atmosphereor an empty chamber after passing through the downstream valve 114,while, in others, the assessment flow 116 is returned to the primaryconduit 106.

The various illustrative logical blocks, modules, and circuits describedin connection with the embodiments disclosed herein may be implementedor performed with general purpose processors in connection with RAM andnon-transitory memory as depicted in FIG. 6, and/or an applicationspecific integrated circuit (ASIC), a field programmable gate array(FPGA) or other programmable logic device, discrete gate or transistorlogic, discrete hardware components, or any combination thereof designedto perform the functions described herein. A general-purpose processormay be a microprocessor, but in the alternative, the processor may beany conventional processor, controller, microcontroller, or statemachine. A processor may also be implemented as a combination ofcomputing devices, e.g., a combination of a digital signal processor(DSP) and a microprocessor, a plurality of microprocessors, one or moremicroprocessors in conjunction with a DSP core, or any other suchconfiguration.

The steps of a method or algorithm described in connection with theembodiments disclosed herein may be embodied directly in hardware, in asoftware module executed by a processor (e.g., as shown in FIG. 6), orin a combination of the two. A software module may reside innon-transitory processor readable mediums such as RAM memory, flashmemory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, aremovable disk, a CD-ROM, or any other form of storage medium known inthe art. An exemplary storage medium is coupled to the processor thatcan read information from, and write information to, the storage medium.In the alternative, the storage medium may be integral to the processor.The processor and the storage medium may reside in an ASIC.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

In conclusion, the present invention provides, among other things, asystem and method for controlling a mass flow rate of a fluid. Thoseskilled in the art can readily recognize that numerous variations andsubstitutions may be made in the invention, its use and itsconfiguration to achieve substantially the same results as achieved bythe embodiments described herein. Accordingly, there is no intention tolimit the invention to the disclosed exemplary forms. Many variations,modifications and alternative constructions fall within the scope andspirit of the disclosed invention as expressed in the claims.

What is claimed is:
 1. A mass flow control system comprising: a primaryconduit for directing a flow of a gas; an adjustment system coupled tothe primary conduit via a secondary conduit, the adjustment system isconfigured to divert a portion of the gas from the primary conduit tothe secondary conduit and provide an adjustment signal that changes whena composition of the gas changes, the adjustment system including: apressurization system to produce an assessment flow in the secondaryconduit that has a changing pressure; a rate-of-change flow meter toprovide, based upon a rate of change of a pressure of the assessmentflow, a first measure of an assessment flow rate of the assessment flow;a mass flow meter disposed to provide a second measure of the assessmentflow rate of the assessment flow through the secondary conduit; anadjustment component to generate the adjustment signal based upon adifference between the first measure and the second measure of theassessment flow rate; a mass flow controller operatively coupled to theprimary conduit to control a primary flow rate of the gas, wherein themass flow controller includes: a valve disposed in the primary conduitto control the primary flow rate of the gas through the primary conduit;a mass flow measurement system configured to generate a measured-flowsignal based upon the primary flow rate of the gas through the primaryconduit; a correction module configured to adjust the measured-flowsignal with the adjustment signal to generate a gas-corrected flowsignal; and a controller configured to control the valve so thegas-corrected flow signal indicates the primary flow rate of the gasthrough the primary conduit is equal to a set point.
 2. The mass flowcontrol system of claim 1, wherein the mass flow controller has anoperating range of at least 100 liters per minute.
 3. The mass flowcontrol system of claim 1, wherein the pressurization system includes:an upstream valve and a downstream valve disposed within the secondaryconduit, wherein the mass flow meter and the rate of change meter aredisposed between the upstream valve and the downstream valve; whereinthe pressurization system includes an assessment-flow controllerconfigured to: open the upstream valve to divert the portion of the gasfrom the primary conduit to the secondary conduit and close thedownstream valve to pressurize the gas; close the upstream valve whilethe downstream valve is closed to trap the pressurized gas in a portionof the secondary conduit that includes a known volume; and open thedownstream valve while the upstream valve is closed to produce theassessment-flow that has the changing pressure.
 4. The mass flow controlsystem of claim 3, wherein the assessment-flow controller includes aprocessor and non-transitory memory encoded with instructions to openand close the upstream and downstream valves to produce the assessmentflow.
 5. The mass flow control system of claim 3, wherein theassessment-flow controller includes hardware to open and close theupstream and downstream valves to produce the assessment flow, whereinthe hardware includes one or more of an FPGA, an ASIC, a programmablelogic device, and discrete gate or transistor logic devices.
 6. A methodfor controlling a flow rate of a gas, the method comprising: controllingthe flow rate of the gas through a primary conduit with a mass flowcontroller; diverting a portion of the gas to a second conduit;producing an assessment flow in the second conduit that has a changingpressure; calculating a first measure of a flow rate of the assessmentflow based upon a rate-of-change of the changing pressure of theassessment flow; measuring the flow rate of the assessment flow with amass flow meter that is affected by the composition of the gas toproduce a second measure of the assessment flow; generating anadjustment signal based upon a difference between the first measure andthe second measure of the assessment flow rate; adjusting ameasured-flow signal of the mass flow controller with the adjustmentsignal to generate a gas-corrected flow signal; and controlling a valveof the mass flow controller so the gas-corrected flow signal indicatesthe flow rate of the gas through the primary conduit is equal to a setpoint.
 7. The method of claim 6, wherein controlling the flow rate ofthe gas through a primary conduit with a mass flow controller includescontrolling the flow rate of the gas to be at least 100 liters perminute.
 8. The method of claim 6, wherein producing the assessment flowincludes: opening an upstream valve in the second conduit to divert theportion of the gas from the primary conduit to the secondary conduit andclosing the downstream valve to pressurize the gas; closing the upstreamvalve while the downstream valve is closed to trap the pressurized gasin a portion of the secondary conduit that includes a known volume; andopening the downstream valve while the upstream valve is closed toproduce the assessment-flow that has the changing pressure.